Bacillus thuringiensis is a Gram-positive spore forming soil bacterium characterized by its ability to produce crystalline inclusions that are specifically toxic to certain orders and species of insects, but are harmless to plants and other non-targeted organisms. For this reason, compositions including Bacillus thuringiensis strains or their insecticidal proteins can be used as environmentally-acceptable insecticides to control agricultural insect pests or insect vectors for a variety of human or animal diseases.
Crystal (Cry) proteins (delta-endotoxins) from Bacillus thuringiensis have potent insecticidal activity against predominantly Lepidopteran, Hemipteran, Dipteran, and Coleopteran larvae. These proteins also have shown activity against Hymenoptera, Homoptera, Phthiraptera, Mallophaga, and Acari pest orders, as well as other invertebrate orders such as Nemathelminthes, Platyhelminthes, and Sarcomastigorphora (Feitelson (1993) The Bacillus Thuringiensis family tree. In Advanced Engineered Pesticides, Marcel Dekker, Inc., New York, N.Y.) The crystal protein does not exhibit insecticidal activity until it has been ingested and solubilized in the insect midgut. The ingested protoxin is hydrolyzed by proteases in the insect digestive tract to an active toxic molecule. (Höfte and Whiteley (1989) Microbiol. Rev. 53:242-255). This toxin binds to apical brush border receptors in the midgut of the target larvae and inserts into the apical membrane creating ion channels or pores, resulting in larval death.
In addition to the endotoxins, B. thuringiensis also produces secreted insecticidal proteins during its vegetative growth stage, namely, vegetative insecticidal proteins (Vip). Since the discovery of the first Vip toxin, two major groups of Vip toxins have been identified in B. thuringiensis. One group of Vip toxins consists of binary toxins which are made of two components, Vip1 and Vip2 (Warren (1997) In N. B. Carozzi and M. G. Koziel (ed.), Advances in insect control: the role of transgenic plants. Taylor & Francis, London, United Kingdom). The combination of Vip1 and Vip2 is highly insecticidal to an agriculturally important insect, the western corn rootworm (Diabrotica virgifera), but does not show any insecticidal activity for any lepidopteran insects (Han et al. (1999) Nat. Struct. Biol. 6:932-936). The other group consists of Vip3 toxins, which share no sequence similarity to Vip1 or Vip2. The first-identified Vip3 toxin, Vip3Aa1, is highly insecticidal to several major lepidopteran pests of maize and cotton, including the fall armyworm Spodoptera frugiperda and the cotton bollworm Helicoverpa zea, but shows no activity against the European corn borer Ostrinia nubilalis, a major pest of maize (Estruch et al. (1996) Proc. Natl. Acad. Sci. USA 93:5389-5394). The deletion of the vip3Aa1 gene from a B. thuringiensis strain resulted in a significant reduction of the insecticidal activity of that B. thuringiensis strain, suggesting that Vip3 contributes to the overall toxicity of B. thuringiensis strains (Donovan et al. (2001) J. Invertebr. Pathol. 78:45-51). It was also observed that Vip3Aa1 kills insects by lysing insect midgut cells (Yu et al. (1997) Appl. Environ. Microbiol. 63:532-536) via cell membrane pore formation (Lee et al. (2003) Appl. Environ. Microbiol. 69:4648-4657).
The intensive use of B. thuringiensis-based insecticides has already given rise to resistance in field populations of the diamondback moth, Plutella xylostella (Ferre and Van Rie (2002) Annu. Rev. Entomol. 47:501-533). The most common mechanism of resistance is the reduction of binding of the toxin to its specific midgut receptor(s). This may also confer cross-resistance to other toxins that share the same receptor (Ferre and Van Rie (2002)).